1. Field of the Invention
The present invention relates to disk drives for computer systems. More particularly, the present invention relates to a disk drive computing repeatable runout (RRO) while the actuator arm is pressed against a crash stop.
2. Description of the Prior Art
Repeatable runout (RRO) in a disk drive is a disturbance in the servo system which can degrade performance by disrupting the head's centerline tracking during read and write operations. The RRO disturbance is typically caused by physical imperfections in the disk drive such as spindle motor runout, disk slippage, disk warping, media defects, and imperfections in the electromechanical servoing mechanism including the mechanism for writing embedded servo sectors onto the disk during manufacturing. Because the imperfections that cause RRO are relatively static, RRO is a predictable disturbance that is periodic with the rotation of the disk. It is known in the industry to estimate and cancel out the periodic RRO disturbance by introducing a feed-forward compensation signal into the servo loop.
The RRO disturbance due to the disk having a non-centric alignment with the spindle motor is sinusoidal with a period equal to the rotation of the disk. This sinusoidal disturbance can be represented as:a*cos (2πk/N)+b*sin (2πk/N)where {a,b} are coefficients corresponding to the magnitude of the disturbance (magnitude of the non-centric offset) and k is an index representing one of N servo sectors.
Prior art techniques typically estimate the composite RRO disturbance due to all sources of eccentricity, such as eccentricities of the servo sectors (written-in RRO), spindle motor runout, disk warping, as well as the RRO disturbance due to the disk's non-centric alignment with the spindle motor. The prior art techniques for estimating the composite RRO disturbance typically involves processing the position error signal (PES) over many revolutions of the disk in order to average out other noise sources (the non-repeatable runout). The composite RRO disturbance is typically estimated during a manufacturing process, and may be updated every time the disk drive is powered on to account for changes that occur over time, particularly disk slippage due to external physical shocks. However, estimating the composite RRO disturbance at every power-on is undesirable in portable applications since it consumes a significant amount of power due to the numerous revolutions. In addition, estimating the composite RRO disturbance over multiple revolutions precludes updating the RRO estimate during normal operation of the disk drive, for example, if the disk drive is subjected to a physical shock causing disk slippage while transferring data to the host. This can lead to an undesirable pause in streaming applications, such as in portable music players, while the disk drive re-estimates the composite RRO disturbance.
There is, therefore, a need for a fast, power-efficient technique for estimating the RRO disturbance when a disk drive is subjected to a physical shock causing disk slippage.